entasis

entasis (ĕnˈtəsĭs) [key] [Gr., = stretching], the slight convex curvature of a classical column that diminishes in diameter as it rises. This device, as used by Greek builders, was of extreme subtlety, the freehand curvature being merely sufficient to guard the contours of the column from any appearance of inward sagging. In the Doric columns of the Parthenon, 34 ft (10.3 m) high and 6 ft 3 in. (1.9 m) in diameter at the bottom, the total convexity amounts to only 3/4 in. (1.91 cm). In Greek Doric columns the entasis began at the foot, but in the Roman orders it was confined to the upper two thirds of the column.